In recent years, along with progress of high-integration and downsizing of semiconductor chips, heat generation amount of such semiconductor chip simply increases, and efficient dissipation of such heat has become a problem to be solved. For this purpose, as a circuit substrate for power module, a circuit substrate is used, which comprises a ceramic substrate such as an aluminum nitride substrate or a silicon nitride substrate having high insulation property and high thermal conductivity, and a metal circuit made of copper or aluminum formed on the ceramic substrate.
A typical heat radiation structure of conventional circuit substrate is that a base plate is soldered to a rear surface (heat radiation surface) of a circuit substrate via a metal plate such as a copper plate, and such a base plate is made of copper. However, in this structure, there has been a problem that when a heat load is applied to a semiconductor device, a crack of solder layer due to the difference of thermal expansion coefficient between the base plate and the circuit substrate occurs, and as a result, heat radiation becomes insufficient to cause malfunction or destruction of a semiconductor chip.
To cope with such a problem, an aluminum alloy-silicon carbide composite is proposed, which has a thermal expansion coefficient close to that of a circuit substrate (Patent Document 1).
Patent Document 1: JP-A-3-509860
A base plate is used as it is joined with a heat-dissipation fin in most cases, and the shape and bow of is the joined portion are also important properties. For example, when a base plate is joined with a heat-dissipation fin, usually a heat-dissipation grease having high thermal conductivity is applied to portions to be joined, and the base plate is fixed to e.g. a heat-dissipation fin or a heat-dissipation unit by screws through holes provided in the peripheral portion of the base plate. However, if many fine irregularities are present on the base plate, a gap is formed between the base plate and the heat-dissipation fin, and even if a heat-dissipation grease having high thermal conductivity is applied, thermal conductivity is significantly decreased. As a result, there has been a problem that heat radiation property of entire module constituted by the ceramic circuit substrate, the base plate and the heat-dissipation fin, etc. is significantly decreased.
To cope with this problem, in order to reduce such a gap between the base plate and the heat-dissipation fin as much as possible, a base plate formed to have a convex bow in advance is used. This bow is obtainable by applying a pressure to the base plate as it is heated, by using a jig having a predetermined shape. However, when the base plate surface is wavy, there has been a problem that the bow shape and its quality are not consistent. Further, due to the variation of bow shape or surface irregularities, there has been a problem that a large gap is still formed between the base plate and the heat-dissipation fin.
There is a method of forming a bow by machining the base plate surface. However, in this method, since the aluminum-silicon carbide composite is extremely hard, significant amount of grinding by a tool such as a diamond is required, and there has been a problem that its cost is high.
To solve the above problem, a method is proposed, in which a flat silicon carbide porous body is infiltrated with a metal containing aluminum as the main component, an aluminum alloy layer made of a metal containing aluminum as the main component is formed on each primary plane, and the aluminum alloy layer in the heat radiation side is mechanically fabricated.
However, in a case of producing a base plate by using this method, the thickness of each surface of aluminum alloy layer after the machining of the base plate is thick. For this reason, the thermal expansion coefficient of the base plate itself increases, and when it is soldered with a ceramic circuit substrate at a time of assembling a power module, there has been a case where a gap is formed on a heat radiation plane that corresponds to a rear surface of the ceramic circuit substrate.
Further, in this method, since it is necessary to control the thickness of the aluminum alloy layer on each primary plane to have uniform thickness and not to make the aluminum-silicon carbide composite exposed to the outside, there has been a problem that high degree of fabrication technique is required.